1. Field of the Invention
The present invention relates to a cylindrical lens system, and more particularly, to a cylindrical lens system for use in a laser beam scanning apparatus.
2. Description of the Prior Art
A laser beam scanning apparatus is generally provided with a rotating polygonal mirror for deflecting a laser beam. Moreover, a laser beam scanning apparatus is known which is provided with a cylindrical lens system for converging a laser beam into a linear form on a reflecting surface of a polygonal mirror in order to correct an inclination of the reflecting surface of the polygonal mirror. In such a laser beam scanning apparatus, a laser beam is converged on the reflecting surface of the polygonal mirror with respect to a direction perpendicular to a scanning surface. The scanning surface means a surface formed by a time-series congregation of the beams deflected by the polygonal mirror. The reflecting surface of the polygonal mirror and a scanned object are in a substantially conjugate relation with respect to a lens system therebetween in the direction perpendicular to the scanning surface. As a result, even if the reflecting surface of the polygonal mirror is inclined, an incident position of a laser beam on the scanned object never shifts.
When a semiconductor laser is used as a beam source, the oscillation wavelength varies according to changes in an environmental condition such as temperature. Moreover, a laser scanning apparatus is known where a plurality of laser beams having different wavelengths are used. In these cases, it is required to correct chromatic aberration generated due to variation or difference in wavelength. This correction is necessary particularly when the spot diameter on a scanned object is small, since the spot diameter is apt to vary due to chromatic aberration.
In an optical system for use in a laser beam scanning apparatus, a collimator lens and an f.theta. lens are known where chromatic aberration is corrected. Cylindrical lens systems including a plurality of lens elements are disclosed by U.S. Pat. No. 4,963,900 and U.S. Pat. No. 5,048,028 (corresponding to Japanese laid-open Patent Application H3-154018).